Process which consists in causing chromium to diffuse



Patented Sept. 9, 1941 PROCESS WHICH CONSISTS IN CAUSING CHROMIUM TO DIFFUSE Karl Daeves and Gottfried Becker, Dusseldorf, and Fritz Steinberg, Krefeld, Germany No Drawing.

Application March 20, 1939, Serial No. 263,060. In Germany March 23, 1938 Claims.

this invention relates to the introduction of chromium into iron and steel articles by diffusion. One method of doing this consists in packing the articles in containers with compounds or alloys of chromium, for example ferrochrom-e, and substances, for example barium chloride, which enter into gaseous combination with the chromium at temperatures of about 900 to 1100 C., and heating the containers. Another method, which is the best so far developed, is to cause a gaseous chromium compound, for example chrom um chloride, to flow over the articles at temperatures of about 900 to 1100 C.

It has been found that the chromium layers formed on the surfaces of the article are excellent only if the articles are made from iron or steel alloys containing less than 0.1% carbon and are comparatively thin. With higher carbon contents the depths to which the chromium layers penetrate are small, and even with carbon contents less than 0.1% satisfactory results are not obtained at the usual temperatures and with the usual periods of diffusion if the articles are thick.

An important object of this invention is to improve the chromium diffusion, whatever the carbon content of the alloy.

Another object of the invention is to provide articles more suitable for chromium diffusion than hitherto. I

It has now been found that, to produce layers containing mixed crystals of iron and chromium that are thick enough to be really useful from both the physical and chemical point of view, it is necessary to take into account not only the proportion of carbon but-also the total amount of the carbon in the article, or at least in the section of it beneath the surface under treatment. It has most surprisingly been found that, as the chromium penetrates into the surface of the article, migration of the carbon from the insidev to the surface takes place, and this migration is the stronger, the greater the carbon content and the thicker the article. Even with iron alloys comparatively low in carbon, the surface layers that can be produced are not very satisfactory from the physical and chemical point of view if the wall thickness of the article exceeds a certain size.

Basing ourselves on this discovery, we use as the alloys from which the iron or steel articles are made alloys containing one or more elements having an affinity for carbon greater than that of chromium. Advantageously the alloys may contain titanium in an amount between 0.1 and 1.0%, but other elements that may be used are columbium, tantalum, zirconium and any similar element having the power of binding the carbon. For the best results, so much of the said element or elements should be used that the migration of carbon throughout the crosssection of the article during the diffusion process is substantially wholly suppressed.

We have further discovered that the product H of the carbon content C of the article in percent and the thickness d of the article in millimetres is an important factor. If this product H (=C.d) exceeds 0.12 when the diffusion is carried on under the usual conditions and for the usual time, i. e. for from 5 to 6 hours at 900 to 1100 C., satisfactory layers cannot be obtained with ordinary carbon steels. Accordingly it is for articles with a value of H greater than 0.12 that it is important to use steels containing titanium or equivalent element. Naturally, the greater the value of H, the greater should be the titanium or equivalent content. The amount of any given element to be used depends upon its aflinity for carbon and the tendency of its carbides to go into solid solution under the conditions of the diffusion.

The action of the titanium or equivalent element can be increased by ensuring that the proportions of those constituents of the alloys that are often called impurities are appropriate. In particular, it is desirable that the silicon content should be between 0.1 and 0.5%, the phosphorus content more than 0.03% and the manganese content not more than 0.3%. Moreover, aluminium contents of more than 0.005% seem to lead to good results.

We claim:

1. Method of producing carbon-containing ferrous metal articles having surfaces containing diffused chromium, which comprises substantially suppressing the migration of carbon, by including, in quantity suflicient to bind all the carbon, a carbon-binding alloy element of greater affinity for carbon than chromium, in a carbon-containing ferrous metal composition in which the carbon content in per cent multiplied by the thickness of the article in millimeters exceeds 0.12, producing from such composition the ferrous metal article, and diffusing chromium into the surface of the article.

2. Method of producing carbon-containing ferrous metal articles having surfaces containing diffused chromium, which comprises including from 0.1 to 1.0% titanium in a carbon-containing ferrous metal composition in which the carbon content in per cent multiplied by the thickness of the article in millimeters exceeds 0.12, producing the ferrous metal article from such composition, and diffusing chromium into the surface of the article.

3. A method according to claim 1, in which from 0.1 to 0.5% silicon is also included in the ferrous metal compositiom 4. A method according to claim 1, in which greater than 0.03% phosphorous is also included in the ferrous metal composition.

5. A method according to claim 1, in which more than 0.005% aluminum is also included in 5 the ferrous metal composition. 

